Personal audio devices, including wireless telephones, such as mobile/cellular telephones, cordless telephones, mp3 players, and other consumer audio devices, are in widespread use. Such personal audio devices may include circuitry for driving a pair of headphones or one or more speakers. Such circuitry often includes a power amplifier for driving an audio output signal to headphones or speakers, and the power amplifier may often be the primary consumer of power in a personal audio device, and thus, may have the greatest effect on the battery life of the personal audio device. In devices having a linear power amplifier for the output stage, power is wasted during low signal level outputs, because the voltage drop across the active output transistor plus the output voltage will be equal to the constant power supply rail voltage. Therefore, amplifier topologies such as Class-G and Class-H are desirable for reducing the voltage drop across the output transistor(s) and thereby reducing the power wasted in dissipation by the output transistor(s).
In order to provide a changeable power supply voltage to such a power amplifier, a direct-current to direct-current power supply (e.g., a boost converter, buck converter, other power converter) or charge pump power supply may be used, such as that disclosed in U.S. patent application Ser. No. 11/610,496 (the “'496 Application”), in which an indication of the signal level at the output of the circuit is used to control the power supply voltage. The above-described topology may raise the efficiency of the audio amplifier, in general, as long as periods of low signal level are present in the audio source.
In embodiments in which the audio amplifier has a differential output stage, the power amplifier may suffer from clipping when the changeable power supply voltage changes from one voltage level to another. Such clipping may occur as circuitry for generating a common-mode voltage of the supply voltage to the audio amplifier (e.g., a common-mode voltage generator or a common-mode feedback loop) may not effectively track power supply voltage transitions, as such circuitry may be optimized in normal operation for high power supply rejection ratio and low power consumption, meaning such circuitry may slowly track the changing power supply voltage.